Conventionally, in a jet engine or the like for an aircraft, a spline connection structure is used as a structure for connecting a shaft and a rotating body. For example, a rotor of a fan or a rotor of a low-pressure turbine is connected to a shaft by a spline connection structure. The spline connection structure has a structure in which a shaft portion having teeth formed at equal intervals on an outer circumferential surface and a cylindrical portion having grooves into which the teeth of the shaft portion arc fitted formed on an inner circumferential surface are connected. The following Patent Documents 1 to 7 disclose the background art related to a spline connection structure and a spline shaft.